Anti-cavitation tunnel for marine propellers

ABSTRACT

An anti-cavitation tunnel ( 11 ) for a watercraft ( 20 ) has a non-convergent, plain (ie., non-aerofoil) section cowling ( 12 ) with a leading edge ( 13 ) located in or forwardly of a plane including the leading edges of the blades ( 28 ) of the propeller ( 25 ) and a trailing edge ( 14 ) which is located forwardly of the rearmost points ( 27 ) of the blades ( 28 ) of the propeller ( 25 ).

FIELD OF THE INVENTION

[0001] THIS INVENTION relates to propellers for boats and, inparticular, anti-cavitation devices to improve the efficiency of boatpropellers.

BACKGROUND ART

[0002] Marine engineers and boat builders have devoted considerableenergies to establishing principles for creating a range of propellertypes for different tasks in vessels. In addition, various devices havebeen developed to enhance the efficiency of conversion from propellerrotation to thrust delivered to a vessel.

[0003] U.S. Pat. No 2,030,375 to Kort discloses a nozzle for locationaround a ship's propeller. This device, and similar, have become widelyknown as Kort nozzles and are suitable for use with screw driven shipsto increase the propulsive efficiency of the propeller. However, thisnozzle is primarily of use at low-revolution-speed of propellers and, inparticular, in large vessels. The nozzle has proven particularlysuitable for tug boats in providing maximal bollard pull and increasedlow speed thrust. When used on small fast moving vessels andparticularly those propelled by outboard, a Kort nozzle may act as adrag when the vessel is planing or approaching maximum velocity.

[0004] U.S. Pat. No. 5,906,522 to Hooper discloses a thrust enhancer formarine propellers. The device includes a peripheral ring with an exhaustring concentrically supported within the peripheral ring by a series ofstruts. The struts may further support individual blades. The aim of thedevice is to cause the propulsive force of a marine propeller to bechannelled more generally towards a single rearward direction. It is arelatively complex piece of equipment.

[0005] An even more complex device is disclosed in U.S. Pat. No.4,637,801 to Schultz for a thrust enhancing propeller duct assembly forwatercraft. This duct has two coaxial cowlings which are staggeredrelative to each other. They are also nozzled to create a venturieffect. The invention is particularly directed to increasing the towingefficiency of watercraft at low speeds, particularly at periods of highslip, such as takeoff.

[0006] Other prior art devices are disclosed in (a) JP 58-126288A(MITSUI ZOSEN KK); (b) DE 4223570 C1 (SCHINEEKLUTH); and (c) JP58-16981A (NIPPON KOKUM KK). JP 58-126288 A (MITSUI) provides aconvergent ring (of aerofoil section) about a propeller with inclinedtips to the vanes. DE 4223570 C1 (SCHNEEKLUTH) provides a ring“diffuser” (4) about the device shaft forwardly of, and of reduceddiameter relative to, the propeller (1) JP 58-126288A (NIPPON) providesa convergent ring (2) about the propellor (1), the ring (2) having anaerofoil section, where respective aerofoil sections (21, 22) areseparated by a slot.

[0007] The above devices are, in general, directed towards increasedefficiency at low speed and vessel takeoff.

[0008] It would be of advantage to provide a thrust enhancinganti-cavitation tunnel that was effective at both low and high propellerrevolution speeds. It would also be advantageous if such a device wererelatively simple to manufacture and install.

[0009] It would also be advantageous if the device could protect thepropellor against damage, eg., due to striking the sea/river bed, rocks,reefs, etc.; and/or to protect marine creatures against propellerstrikes. Examples of such protective devices are disclosed in (a) EPO433510 A1 (TABRAT); (b) U.S. Pat. No. 5,176,550 (HOOPER); (c) U.S. Pat.No. 2,983,246 (MANLEY); and (d) WO 93/17907 (TAYLOR).

OBJECT OF THE INVENTION

[0010] It is an object of the present invention to overcome or at leastameliorate one or more of the above disclosed deficiencies in the priorart.

[0011] Other preferred objects will become apparent from the followingdescription.

SUMMARY OF THE INVENTION

[0012] In one form, although it need not be the only or indeed thebroadest form, the invention lies in an anti-cavitation tunnel for apropeller of a watercraft, said anti-cavitation tunnel comprising:

[0013] a cowling having a leading edge and a trailing edge;

[0014] wherein the maximum distance between the leading edge and thetrailing edge of the cowling is less than the maximum depth of a vane ofthe propeller.

[0015] In a further form, the invention resides in an anti-cavitationtunnel for use with a propeller of a watercraft, said anti-cavitationtunnel comprising:

[0016] a cowling having a leading edge and a trailing edge; and

[0017] attachment means for securing the cowling in an operatingposition in relation to the propeller;

[0018] wherein the maximum distance between the leading edge and thetrailing edge of the cowling is less than the maximum depth of a vane ofthe propeller.

[0019] Preferably, the cowling is non-convergent along its length and isof plain (ie., non-aerofoil) section.

[0020] The trailing edge may be planar or describe at least one rearwardprojection.

[0021] Preferably, the trailing edge may describe a series of continuouscurved rearward projections. Preferably, the projections are indiametrically opposed pairs. The trailing edge may describe a continuoussine or wave pattern.

[0022] Preferably, the attachment means comprises at least one bracketengageable with an outboard motor. Suitably, the operating position issuch that the leading edge of the cowling is located in the vicinity ofa plane including an anterior surface of the propeller. The attachmentmeans may be in the form of integral moulding. The attachment means maybe plastic welding.

[0023] Most preferably, the leading edge is substantially located at, orrearwardly of, the plane including the anterior surface of thepropeller.

[0024] Preferably, the maximum distance between the leading edge and thetrailing edge of the cowling is in the range of ½ to ¾ of the depth of avane of the propeller. Most preferably, the maximum distance between theleading and trailing edge is ⅔ of the depth of a vane of the propeller.

[0025] Preferably, the lower portion of the cowling enclosesapproximately one-half to three-quarters of the length of the propellerand an upper portion of the cowling encloses substantially all of thetop of the propellor and tapers down to enclose approximately one-halfto three-quarters of the propeller.

[0026] The anti-cavitation tunnel may further comprise a fin located inan upper region of the cowling.

[0027] The anti-cavitation tunnel may also comprise a transition zonebetween the trailing edge and an upper bracket.

[0028] In a further aspect, the invention resides in a method ofimproving the efficiency of a propeller of a watercraft, said methodinclude the step of:

[0029] installing an anti-cavitation tunnel according to any one of theabove described embodiments in an operative position in relation to thepropeller.

[0030] The method may further include the step of installing theanti-cavitation tunnel so that a leading edge of the tunnel issubstantially in the same plane as an anterior surface of the propeller.

[0031] In yet a further aspect, the invention may reside in a method ofimproving the efficiency of a propeller of a watercraft, said methodincluding the step of:

[0032] locating an anti-cavitation tunnel so that it shrouds an anteriorportion of the propeller and exposes a rearward portion of thepropeller.

[0033] Preferably, the shrouded anterior portion is equal to or largerthan the exposed rearward portion when calculated on the depth of thepropeller.

[0034] Most preferably, the anterior portion is twice as large as theexposed rearward portion when calculated on the depth of a vane of thepropeller.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a perspective view of a first embodiment of ananti-cavitation tunnel according to the present invention;

[0036]FIG. 2 is a side view of the anti-cavitation tunnel of FIG. 1attached to an outboard motor;

[0037]FIG. 3 is a perspective view of the anti-cavitation tunnel andoutboard motor of FIG. 2;

[0038]FIG. 4 is a schematic side view of the anti-cavitation tunnel ofthe present invention showing water flow during use;

[0039]FIG. 5 is a schematic top view of the anti-cavitation tunnel ofFIG. 4 showing water flow during use;

[0040]FIG. 6 is a rear view of a second embodiment of theanti-cavitation tunnel;

[0041]FIG. 7 is a side view of the anti-cavitation tunnel of FIG. 6attached to an outboard motor;

[0042]FIG. 8 is a rear view of a third embodiment of the anti-cavitationtunnel;

[0043]FIG. 9 is a side view of the anti-cavitation tunnel attached to anoutboard motor;

[0044]FIG. 10 is a rear view of a fourth embodiment of theanti-cavitation tunnel;

[0045]FIGS. 11 and 12 are respective side and top plan views of theanti-cavitation tunnel of FIG. 10 attached to an outboard motor;

[0046]FIG. 13 is a rear view of an adjustable fin which may be used withthe anti-cavitation tunnels of FIGS. 1 to 12;

[0047]FIGS. 14 and 15 are respective side and top plan views of theadjustable fin of FIG. 13 attached to an outboard motor.

DETAILED DESCRIPTION OF THE DRAWINGS

[0048] Referring to FIG. 1, there is shown a first embodiment of theanti-cavitation tunnel 11 comprising a substantially circular cowling 12with a leading edge 13 and trailing edge 14. The cowling 11 isnon-convergent along its length and is of plain (ie., non-aerofoil)section. The cowling 12 has an upper attachment bracket 15 and lowerattachment bracket 16. The anti-cavitation tunnel 11 also includes a fin17 located just aft of upper attachment bracket 15 in an upper region ofthe cowling 12.

[0049] Referring to FIG. 2, there is shown the anti-cavitation tunnel 11affixed to an outboard motor 18 which, in turn, is attached to the stern19 of a boat 20.

[0050] The anti-cavitation tunnel 11 is attached by a bolt 21 passingthrough an aperture in lower attachment bracket 16 and through the guideskeg 22. The anti-cavitation tunnel 11 is also attached to thecavitation plate 23 of the outboard motor 18 by bolts 24 passing throughapertures drilled in the cavitation plate 23 and into upper attachmentbracket 15. As can be seen in this view and importantly for theinvention, the cowling 12 does not completely cover the propellor 25 ina fore and aft direction. The leading edge 13 is located level with oraround a plane including the propellor leading edge or anterior surface26. The trailing edge 14, however, is forward of the rearmost points 27of the propellor vanes 28.

[0051] The attachment means may be any suitable means known to a skilledaddressee. For example, the cowling 12 may be integrally moulded withthe outer casing of the outboard leg. Alternatively, plastic welding maybe used to attach the cowling. A frame may be used to hold the cowling11 in position relative to the propellor 25. the cowling may be formedor continuous with any appropriate surrounding structure, for example,the hull of a vessel with a fixed propellor.

[0052] The propellor vanes 28 are supported by a central hub 29 which,in turn, is engaged with a drive shaft (not shown).

[0053] The maximum distance between the leading edge 13 of a cowling andthe trailing edge 14 of the cowling is the distance “a”. The maximumlength of the propellor vanes 28 is “b” which is a fore and aftmeasurement of a vane of the propellor. The inventor has discoveredthat, importantly, the distance “a” must be less than the distance “b”for the anti-cavitation tunnel to work effectively. Preferably, “a” isin the range of ½ to ¾ of “b”. Most preferably, “a” is in the range of ½to ⅔ of “b”.

[0054] Typically, the radial clearance between the cowling 12 and theblades 28 of the propellor 25 (for an outboard motor in the 10-140 HPrange) will be in the range of 10-30 mm.

[0055] The scalloped trailing edge 14 is shown as a regular sinusoidalcurve around the edge. The trailing edge 14 may, in fact, have just asingle rearward projection rather than multiple projections as shown.Preferably, however, there are at least two rearward projectionscreating a curved trailing edge and those projections are ideallylocated to balance the action of the cowling. A plurality ofprojections, as shown, may be used wherein the projections are arrangedin diametrically opposed pairs, thereby creating balance in the functionof the anti-cavitation tunnel 11. Although a regular curve is shown, itis clear to a skilled addressee that other geometric shapes will besuitable for the function of the anti-cavitation tunnel and any suitableconfiguration may be adopted including shapes that are angular. It isalso clear to a skilled addressee that, in fact, the trailing edge 14may be an even edge substantially parallel to leading edge 13 giving aring effect to the cowling 12.

[0056] This view also shows an upward arc 30 of cowling 12 to meet upperattachment bracket 15, thereby forming a transition zone between cowling12 and bracket 15. This feature provides an additional advantage in thatit creates lift during water flow through anti-cavitation tunnel 11which increases the efficiency of thrust delivery by the outboard motor18 leading to the boat 20 rising onto the plane quicker and moreefficiently. This action is enhanced by the presence of fin 17 whichprojects into the water flow through the cowling 12 and furtherincreases the provision of lift to the motor 18 and efficiency ofplaning of the boat 20.

[0057] Referring to FIG. 3, there is shown a perspective view ofanti-cavitation tunnel 11 of the invention located on an outboard motor.The cowling 12 is attached via bolt 21 and upper bolts 24 to the skeg 22and cavitation plate 23, respectively. The propeller vanes 28 areattached to central hub 29 which, in turn, is attached to a drive shaft(not shown) by retention nut 31. Leading edge 13 describes an intakeopening 32 through which water enters in use.

[0058] Referring to FIG. 4, there is shown a schematic vector diagramfor water flow through the anti-cavitation tunnel 11 when viewed fromthe side. As the boat, motor and anti-cavitation tunnel 11 pass throughthe water, water flows in the direction of the intake arrows 33. Theflow around fin 17 is shown by the arrows at 34 with a resultant lift tothe anti-cavitation tunnel 11 and outboard motor. Circular, angularlateral propulsion is provided by the flow from arrows at 35, whereasoutward rear lower propulsion is provided by arrows as shown at 36.

[0059] A further flow schematic is seen in top view in FIG. 5. Again,intake arrows are seen at 33. Circular angular lateral propulsion isseen at 35. Outward lower rear propulsion is shown as 36. An improvedoutward higher rear propulsion is shown at 37 formed by the combinedcircular angular lateral propulsion and outward rear lower propulsion.The invention may use the lateral forces to gain efficiency in a forwardmotion. By maximising the lateral forces to increase efficiency, theanti-cavitation tunnel is intended to channel the lateral thrust towardsthe rear of the propeller. This concentrates the lateral forces to therear of the propeller but also requires a balancing percentage of thepropeller to remain partially exposed, as this allows a more effectiveflow of lateral thrust.

[0060] A pulsing action is created by the wave effect or scalloped shapeof the trailing edge 14 of the cowling 12. It is believed this uses themaximum lateral force to create a pulsing effect which improves thelateral propulsion and helps to improve performance. As noted above,however, the cowling may be produced without the wave effect on thetrailing edge. A smaller percentage of exposed propeller to a largerfrontward percentage enclosed by the cowling is preferred.

[0061] Referring now to FIGS. 6 and 7, the anti-cavitation tunnel 111,again of non-convergent, plain (ie., non-aerofoil) section, has aleading edges 113 which is inclined at any angle in the range of 5° to15° to the vertical (taken as a plane perpendicular to the axis ofrotation of propellor 125); while the trailing edge 114 is substantiallyvertical but spaced forwardly of, the rearmost points 127 of the vanesof the propellor 125. The cowling 112 is attached to the cavitationplate 125. The cowling 112 is attached to the cavitation plate 123 ofthe guide skeg 122 by four fasteners (not shown) and to the lower end ofthe skeg 122 by a lower skeg bracket 150.

[0062] The inclined leading edge 113 provides improved accelerationcharacteristics as the angle of inclination allows efficient intake flowof the water through the tunnel 111 due to the natural tilt position ofthe boat in the water. The angle in conjunction with the tilt positionreduces drag or resistance.

[0063] The trailing edge 114 is preferably spaced a distance C forwardlyof the rearmost parts of the propellor 1256, where C is preferably ¼ to½ of b, ie., approximately one-quarter to one-half of the lower portionof the propellor 125 is exposed. As shown in FIG. 7, the cowling 112 isconfigured to cover all of the top of the propellor 125 and thencascades down to cover one-half to three-quarters of the propellor 125in the upper half (ie., one-quarter to one-half of the propellor 125 isexposed).

[0064] It has been found by experimentation that the two cascadingsurface areas described above (and illustrated in FIG. 7) maximises theinternal forces. The use of the two specific areas has shown thepressurisation of two separate surface areas decreases cavitation by alarge percentage.

[0065] Because of the partial exposure of the propellor, and due to thecascading surface areas, there is little, or no, loss of top speed.Using the larger cascading surface area at the top of the cowling 12produces an unevenly distributed directional flow and also produces aneffective and instantaneous lift of the boat onto the plane.

[0066] For larger boats (eg., working boats) pushing heavier work loads,the cowling 212 of the anti-cavitation tunnel 211 of FIGS. 8 and 9extends only over the upper portion of the propellor 225, the lowerportion being surrounded by a (stainless steel)rod 212 a, attached tothe skeg by a lower skeg bracket 250.

[0067] The rod 212 a protects the propellor 225 against heavy impacts.

[0068] The cowling 212 with its cascading surface area, providesimproved performance over a propeller without the tunnel 211, includingimproved timing ability.

[0069] FIGS. 10 to 12 show a fourth embodiment of the anti-cavitationtunnel 311, which is protected by a cage 370 which totally protects thetunnel 311 (and propeller 325) without diminishing the performanceadvantages provided by the tunnel 311.

[0070] The performance of the boat, fitted with any one of theanti-cavitation tunnels hereinbefore described, can be further improvedby the fitting of an adjustable fin 480, hingedly mounted at the rear ofthe cavitation plate 423 on the skeg 422 and adjustable manually (via aturnbuckle) or mechanically (via a pneumatic or hydraulic ram 481). Thefin 480 co-operates with the tunnel to improve lift generated by thepropellor.

[0071] The anti-cavitation tunnel may be manufactured in any suitablematerial, including stainless steel, aluminium or high density plastic.

[0072] The inventor has found that the anti-cavitation tunnel increasesthe performance of marine craft, including increasing the thrust andfuel efficiency of the drive system and decreasing cavitation around thepropeller (eg., enabling a 70 HP motor to provide the same thrust as astandard 90 HP motor without the tunnel). The invention has alsoprovided increased stability in steering and, due to the channelling ofthe water, appears to quieten the exhaust note of an outboard motor.

[0073] The top end speed of a boat may also be increased and the vesselwill have better turning characteristics when the device is used with anoutboard motor.

[0074] No reduction in reversing characteristics has been noted intesting.

[0075] Although the invention has been primarily described in relationto an outboard motor, it is clear to a skilled addressee that the devicemay be applied to other forms of propeller drive systems, such asinboard motors with drive shafts connected to an external propeller.

[0076] Rubber gaskets may be used in the attachment of the invention toa motor to prevent water leakage into bolt holes and also minimise theeffects of vibration.

[0077] In a further embodiment, the cowling of the anti-cavitationtunnel may be formed by a truncated cylinder with a leading edgeperpendicular to its longitudinal access and a trailing edge angled tothat axis having the widest portion of the cowling at the bottom of thecowling.

[0078] Throughout this specification, the aim has been to describe thepreferred embodiments of the invention without limiting the invention toany one embodiment or specific collection of features.

1. (amended) an anti-cavitation tunnel for a propellor of a watercraft,said anti-cavitation tunnel comprising: a cowling, non-convergent alongits length and of plain (non-aerofoil) section, having a leading edgeand a trailing edge wherein the leading edge of the cowling issubstantially located in, or rearwardly of, the plane including theanterior surface of the propellor, the trailing edge of the cowling issubstantially located in, or forwardly of, the plane including theposterior surface of the propellor, and the maximum distance between theleading edge and the trailing edge of the cowling is less than themaximum fore and aft length of a vane of the propellor.
 2. (Amended) Ananti-cavitation tunnel for use with a propellor of a watercraft, saidanti-cavitation tunnel comprising: a cowling, non-convergent along itslength and of plain (non-aerofoil) section, having a leading edge and atrailing edge; and attachment means for securing the cowling in anoperating position in relation to the propellor; wherein the leadingedge of the cowling is substantially located in, or rearwardly of, theplane including the anterior surface of the propellor, the trailing edgeof the cowling is substantially located in, or forwardly of, the planeincluding the posterior surface of the propellor, and the maximumdistance between the leading edge and the trailing edge of the cowlingis less than the maximum fore and aft length of a vane of the propellor.3. (Amended) A tunnel as claimed in claim 1 or claim 2 wherein: thetrailing edge is planar or describes at least one rearward projection.4. (Amended) A tunnel as claimed in claim 3 wherein: the trailing edgedescribes a series of continuous curved rearward projections, theprojections being in diametrically opposed pairs.
 5. (Amended) A tunnelas claimed in claim 2 wherein: the attachment means comprises at leastone bracket engageable with an outboard motor.
 6. (Amended) A tunnel asclaimed in claim 1 or claim 2 wherein: the maximum distance between theleading edge and the trailing edge of the cowling is in the range of ½to ¾ of the length of a vane of the propellor.
 7. (Amended) A tunnel asclaimed in claim 6 wherein: the maximum distance between the leading andtrailing edge is ⅔ of the length of a vane of the propellor. 8.(Amended) A tunnel as claimed in claim 6 or claim 7 wherein: the lowerportion of the cowling encloses approximately one-half to three-quartersof the length of the propellor and an upper portion of the cowlingencloses substantially all of the top of the propellor and tapers downto enclose approximately one-half to three-quarters of the propellor. 9.(Amended) A tunnel as claimed in any one of claims 1 to 8, and furthercomprising a fin located in an upper region of the cowling. 10.(Amended) A tunnel as claimed in any one of claims 1 to 9, and furthercomprising a transition zone between the trailing edge and an upperbracket.
 11. (Amended) A method of improving the efficiency of apropellor of a watercraft, said method including the steps of:installing an anti-cavitation tunnel according to any one of claims 1 to10 in an operative position in relation to the propellor.
 12. (Amended)A method of improving the efficiency of a propellor of a watercraft,said method including the steps of: locating an anti-cavitation tunnelas claimed in any one of claims 1 to 10 so that it shrouds an anteriorportion of the propellor and exposes a rearward portion of thepropellor.
 13. (Amended) A method as claimed in claim 12 wherein: theshrouded anterior portion is equal to or larger than the exposedrearward portion when calculated on the depth of the propellor. 14.(Amended) A method as claimed in claim 13 wherein: the anterior portionis twice as large as the exposed rearward portion when calculated on thedepth of a vane of the propellor.
 15. (New) An anti-cavitation tunnelfor a propellor of a watercraft, said anti-cavitation tunnel comprising:a cowling, non-convergent along its length and of plain (non-aerofoil)section, having a leading edge and a trailing edge wherein the leadingedge of the cowling is substantially located in, or rearwardly of, theplane including the anterior surface of the propellor and the maximumdistance between the leading edge and the trailing edge of the cowlingis less than the maximum fore and aft length of a vane of the propellor;wherein: the lower portion of the cowling encloses approximatelyone-half to three-quarters of the length of the propellor and an upperportion of the cowling encloses substantially all of the top of thepropellor and tapers down to enclose approximately one-half tothree-quarters of the propellor.
 16. (New) An anti-cavitation tunnel foruse with a propellor of a watercraft, said anti-cavitation tunnelcomprising: a cowling, non-convergent along its length and of plain(non-aerofoil) section, having a leading edge and a trailing edge; andattachment means for securing the cowling in an operating position inrelation to the propellor; wherein the leading edge of the cowling issubstantially located in, or rearwardly of, the plane including theanterior surface of the propellor, and the maximum distance between theleading edge and the trailing edge of the cowling is less than themaximum fore and aft length of a vane of the propellor; and wherein: thelower portion of the cowling encloses approximately one-half tothree-quarters of the length of the propellor and an upper portion ofthe cowling encloses substantially all of the top of the propellor andtapers down to enclose approximately one-half to three-quarters of thepropellor.